A workpiece turned on a lathe has a central axis about which the piece is rotatably driven. It is important to know the diameters of a workpiece that is being machined on a lathe so that it is certain that the workpiece is within the tolerances required. The distance from the axis of a turned workpiece to its outer circumferential surface is the radius of the workpiece at a specific location along its length. The diameter is two times the radius.
Inexpensive, conventional micrometers and calipers are used to measure the diameter, or diameters if there are multiple ones of interest, along the length of a workpiece. However, since mechanical surface contact is required for micrometers and calipers to work, slight fluctuations in surface texture introduce error in measurement. The operator of the micrometers or calipers must also be experienced with them in order to obtain as accurate and repeatable measurements as are possible with them.
Additionally, the contours of lathe-turned workpieces may make micrometers and calipers impossible to use due to the lack of positive engagement between the contacting surfaces of the micrometers and calipers and the workpiece. For example, if the outer surface of the workpiece is not parallel to the axis of the workpiece, as with a conical structure, the surfaces of the micrometer, which are aligned parallel to the axis of the workpiece, will be impossible to orient parallel to the workpiece surface to obtain an accurate measurement.
A prior art gauge which has replaced the micrometers and calipers for some applications has a base into which the workpiece is mounted after being removed from a lathe or grinder. The gauge has a laser beam which it scans along a fan shaped path across a collimator lens. The scanned beam passes through the collimator lens which directs the beam along a collimated path. Over time, the scanned beam forms a planar, sheet shaped beam. The middle portion of the laser beam "plane" strikes the workpiece with some of the lateral edges of the plane passing on both lateral sides of the workpiece. The light passing laterally by the workpiece strikes a collector lens which is on the opposite side of the workpiece as the collimating lens. The collector lens focuses the laterally passing light onto a detector which is on the opposite side of the workpiece as the collimating lens. The workpiece creates a shadow in the scanned planar beam that interrupts the plane of light directed onto the detector and creates a "shadow". The scanning rate of the laser and the amount of time the shadow is present on the detector are relayed to a computer. The computer uses these data and geometric parameters to calculate the diameter of the workpiece.
The conventional laser device is prone to error in many areas. First, when a workpiece is removed from the chuck of a lathe and is placed in the mount to be measured, it is most likely that the axis on which the workpiece is mounted in the laser gauge will not be aligned with the original axis of the workpiece as defined in the lathe. Since the axis of the workpiece is created on the lathe by holding a cutting tool a selected distance from the axis and cutting material away, the axis of the workpiece would be maintained with certainty by leaving the workpiece in the lathe. This disadvantage of moving the workpiece to a second mount is amplified when the need for further altering the workpiece arises, once measurement has taken place. For example, if the workpiece is in a gauge mount and needs to be further machined, it must be removed from the gauge mount and replaced in the lathe. This introduces the likelihood of further offsetting the workpiece from its axis. The need to position the laser light source, light detector and lenses on opposite sides of the workpiece, and the large size and weight of the conventional laser gauge, make it impractical to mount it on a lathe.
Further error is introduced by the conventional laser gauge by the use of lenses and mirrors in the path of the laser beam prior to the laser beam striking the workpiece. Lenses and mirrors are inherently imperfect, and therefore the gauge can have error in the measured value to at least the extent that the lenses and mirrors have imperfection. These imperfections also limit the maximum measurable diameter of workpieces to about two inches.
In U.S. Pat. No. 4,427,296, Demarest et al. disclose a conventionally used laser gauge which scans a collimated laser beam over an object. The prior art cited by Demarest uses a parabolic lens to collimate the scanned beam into parallel beams passing over the object. The parallel beams are then collected and focused by a second parabolic lens which focuses them onto a detector. Demarest, among other things, substitutes mirrors for the lenses.
The error in the conventional laser gauges using lenses and mirrors, is introduced when the lenses or mirrors are between the beam source and the workpiece. When the beam reflects from a mirror or passes through a lens, the beam takes a path which is altered from its theoretical or ideal path. This is due to imperfections inherent in all lenses and mirrors. The imperfectly directed beam can strike the workpiece when it should pass by and be detected. This leads to the computer calculating a measurement based on a false edge of the workpiece. When these imperfections are introduced in the path of the beam prior to the beam contacting the object being measured, significant error is introduced in the measuring device.
The placement of the second mirror and detector show that Demarest teaches to have active circuitry on opposite sides of the object being measured. If the Demarest gauge were mounted on a lathe, as would be desired, then the active circuitry would need to be on both sides.
In U.S. Pat. No. 3,854,052, Asar et al. disclose a gauge using a laser which is scanned over an object along a fan shaped path. The laser beam is then collimated and then collected by a pair of lenses which focus the light onto a photodetector.
The lenses are on the opposite side of the workpiece as the light source which eliminates the error introduced by having an optical element between the light source and the object being measured. However, by placing the lenses, and more specifically the photodetector and video amplifier circuit, on the side of the workpiece opposite the light source, the disadvantage of making the gauge significantly more fragile exists. If the gauge disclosed by Asar were mounted to a lathe as is most desirable, some fragile elements of the gauge will be on both sides of the object. By placing some sort of active circuitry between the object being measured and a worker, the likelihood of damage to the active circuitry becomes significant. The active circuitry includes the photodetector and video amplifier circuit, and the light source which is a laser and a rocking mirror.
There also exist digitally controlled cutting and grinding machines which have a feedback mechanism controlling and recording the position of a cutting tool. While these machines can control the position of a tool, they normally use a predicted position of the tip of the tool with respect to some coordinate, for example the axis of the workpiece. Since the tools wear due to their mechanical contact with the workpiece, the actual placement of the tool tip varies slightly with use.
There is a need for a gauge that will measure a turned workpiece while it is still mounted in a grinder or a lathe. The term "lathe" is used to refer to lathes, grinders and other similar machines. The gauge should be simple to operate with accuracy, and it should not need multiple calibrations for variations in workpiece diameter. The active circuitry of the gauge should not be between the workpiece and a worker, and should be enclosed to prevent damage.